1. Field of the Invention
The present invention relates generally to a method of processing an image, and more particularly, to a method of processing an image using an error diffusion method.
2. Description of the Related Art
An error diffusion method is one of gradation representations when image data represented by values of multiple levels is provided to a printer for a binary value. This is an excellent method in which gradation and resolution are compatible with each other. However, a method of processing an image by extracting a characteristic of an original image and controlling an error diffusion parameter based on the characteristic to carry out processing of an image of high quality and with high accuracy is disclosed in Japanese Patent Layed-Open No. 63-214073.
In the above-described method of processing an image, a characteristic of a pixel of interest (edge, concentration and the like) is extracted, and based thereon an amount of error feedback (in the case of a character, the amount of error feedback is set to 0 for simple conversion to a binary value) is controlled. However, such control has a problem from the standpoint of precision. For example, take into consideration the case where, although a characteristic of a pixel of interest is determined to be a character region, a characteristic of a plurality of pixels in the vicinity of the pixel of interest is determined to be a photography region. In this case, in a conventional method of processing an image, the pixel of interest is converted to a binary value by using an error diffusion parameter as the character region. However, it is appropriate to consider that this is caused by generation of any noise on the pixel of interest. Such noise is not subjected to any appropriate process.
In the above-described conventional method of processing an image, an original image is read out to be A/D converted. After that, shading correction or the like for correcting sensitivity variation of a CCD sensor and lighting variation of a light source is carried out in a correcting circuit. A characteristic of a pixel of interest (edge, concentration and the like) is extracted in a characteristic extracting unit based on data after correction, and based thereon the amount of error feedback is controlled.
However, when data is provided to a printer or a CRT having low resolution of dot density by such a method of processing an image, the dot density is increased in high concentration data. Gradation in a shadow region is worse than a theoretical value because of dot overlapping and the like.
On the other hand, in order to prevent deterioration of the gradation, image data is corrected to extract a characteristic of a pixel based on the corrected data, which, in turn, results in characteristic determination with low precision.
In addition, in the above-described conventional method of processing an image, a characteristic of a pixel of interest (edge, concentration and the like) is extracted, and based thereon an amount of error feedback (in the case of a character, the amount of feedback is set to 0 for simple conversion to a binary value) is controlled. However, only such control is not sufficient from the standpoint of precision. Error data obtained by an error diffusion method includes a positive component and a negative component. Since calculation was carried out for the absolute amount of the error data in the above-described conventional method of processing an image, it was difficult to process a special original image in high quality in which a character is written on a photography intermediate region.